Genetics of Resistance to Pierce’s Disease

A wide range of rootstocks and grape species from across the southern US have been inoculated with Xylella fastidiosa (Xf). IC-PCR tests to detect Xf presence have not been completed, but all the samples have been taken. IC-PCR tests are necessary to fully explain some of the observed reactions. The rootstocks were tested to evaluate their ability to be symptomless hosts, and therefore provide an alternative means of spreading Xf. St. George rootstock may fall in this category, although all of the other V. rupestris selections tested displayed symptoms. Dog Ridge had very mild symptoms and may also be a symptomless carrier. All the other rootstocks had clear symptoms. Accessions from all the southern grape species were tested. Vitis riparia was included because it resides outside of the PD range and results with this species would help qualify reactions with the other species. Two of these accessions were killed by Xf and the other two had severe symptoms. Symptoms ranged from mild to intense among the other species and also varied within a species (different accessions had different results). The Muscadinia species {munsoniana and rotundifolia) generally were healthy and are expected to be the most resistant after IC-PCR testing. Work towards mapping resistance to Xf is also progressing. Forty V. rupestris X M. rotundifolia selections have been tested and three have high levels of resistance (lack of symptoms and limited Xf movement based on IC-PCR results). Unfortunately, the mapping populations that were made previously, 8913-02 and 8913-21 crossed to Chardonnay (reported on in the interim report and last year) cannot be used because neither of these selections is adequately resistant. One of three highly resistant selections is 8909-15. A genetic map was created for Xiphinema index resistance using the 8909-15 X 8909-17 seedling population. This population may also be ideal to map Xf resistance since 8909-17 is susceptible. However, we also plan to produce other seedling mapping populations by crossing 8909-15 and the other resistant selections with Chardonnay. Crosses with Chardonnay will give a more clear range of responses from very resistant to very susceptible and should therefore be easier to map in. These crosses will also begin the introgression of Xf resistance fromM. rotundifolia into V. vinifera.